Few things in the Universe keep the beat as reliably as an atom's pulse.
Yet even the most advanced 'atomic' clocks based on variations of these quantum timekeepers lose count when pushed to their limits.
Physicists have known for some time that entangling atoms can help tie particles down enough to squeeze a little more tick from every tock, yet most experiments have only been able to demonstrate this on the smallest of scales.
A team of researchers from the University of Oxford in the UK have pushed that limit to a distance of two meters (about six feet), proving the mathematics continues to hold true over larger spaces.
Not only could this improve the overall precision of optical atomic clocks, it allows for a level of comparison in the split-second timing of multiple clocks to a degree that could reveal previously undetectable signals in a range of physical phenomena.
As the name indicates, optical atomic clocks use light to probe the movements of atoms to keep time.
Like a child on a swing, components of atoms whizz back and forth under a consistent set of constraints. All that's needed is a reliable kick, such as a photon from a laser, to set the swinging in motion.
Various techniques and materials have been tested over the years to advance the technology to the point that differences in their frequencies barely add up to a second's worth of error over the 13-odd billion years of the Universe – a level of precision that means we might need to rethink the very way we measure time itself.
As fine-tuned as this technology happens to be, there comes a point when the very rules of time-keeping themselves become a little vague thanks to the uncertainties of the quantum landscape that introduce a bunch of catch-22 situations.
For example, higher frequencies of light can improve precision, but comes at the cost of small uncertainties between the photon's kick and the atom's response becoming more important.
These in turn can be ironed out by reading the atom multiple times, a solution not without its own problems.
A 'single shot' reading with the right kind of laser pulse would be ideal. Physicists know that the uncertainty of this approach can be improved if the atom being measured has already had its fate entangled with another.
Entanglement is at once an intuitive and bizarre concept. According to quantum mechanics, objects can't be said to have a value or state until they're observed.
If they're already part of a bigger system – maybe through an exchange of photons with other atoms – all parts of the system will be fated to deliver a relatively predictable outcome.
It's like flipping two coins from the same wallet, knowing if one comes up heads the other will come up tails even as it spins in the air.
The two 'coins' in this case were a pair of strontium ions, entangled with a photon that was sent down a short length of optic fiber.
The test itself didn't produce any revolutionary levels of precision in optic atomic clocks, though it wasn't intended to.
Instead the team showed by entangling the charged atoms of strontium, they could reduce the uncertainty of the measurement under conditions that should allow them to improve precision in the future.
Knowing macroscopic distances of a few meters presents no challenge, it's now theoretically possible to entangle optical atomic clocks around the world to improve their precision.
"While our result is very much a proof-of-principle, and the absolute precision we achieve is a few orders of magnitude below the state of the art, we hope that the techniques shown here might someday improve state-of the art systems," says physicist Raghavendra Srinivas.
"At some point, entanglement will be required as it provides a path to the ultimate precision allowed by quantum theory."
Squeezing a little more confidence out of every tick-tock of an atomic clock could be just what we need to measure tiny differences in time produced by masses over the smallest of distances, a tool that might lead to quantum theories of gravity.
This research was published in Nature.